1. Field of the Invention
The present invention relates to a method of improving magnetic properties of soft magnetic alloys and, more particularly, to a method of annealing crystalline or nanocrystalline magnetic alloys in forms of sheet, ribbon, or thin film under application of an external magnetic field whose direction undergoes a periodic rotation, oscillation, or step-variation in a plane, referred to as a dynamic magnetic field herein, to produce a planar texture in the plane.
2. Description of Related Art
Materials exhibiting good soft magnetic properties (ferromagnetic properties) include certain crystalline alloys in forms of sheet, ribbon, or thin film (such as Permalloys) and certain alloys in forms of sheet, ribbon, or thin film that contain nanocrystalline particles. In order to produce a good soft magnetic material, the composition of the alloy has to be selected such that its magnetocrystalline anisotropy and the magnetostriction of the material are close to zero. Further improvement of soft magnetic properties includes producing a certain crystallographic texture which favors the 180.degree. domain structure. One way to achieve the required texture is magnetic annealing, i.e., annealing the magnetic material in the presence of a magnetic field.
Consider binary transition metal alloys A.sub.100-X B.sub.X. For non-magnetic alloys, the populations of A-A, A-B, and B-B atomic pairs are determined by the composition of the alloy, and their spatial distribution is random. For crystalline and nanocrystalline magnetic alloys, however, during the fabrication or annealing process when the temperatures are below the Curie temperature of the material, the atomic moments are coupled by the exchange interaction thus forming domains, and then the distribution of A-A, A-B, and B-B atomic pairs in the domains become ordered due to the dipolar interaction between the magnetic atoms. This is known as directional ordering. Directional ordering leads to the occurrence of an additional induced uniaxial magnetic anisotropy with a 180.degree. symmetry. This induced anisotropy is the major impediment for further improvement of the soft magnetic properties of crystalline and nanocrystalline alloys.
The approach currently used by manufacturers to reduce the effect of the directional order on the magnetization process is known as static magnetic annealing, i.e., annealing the material in the presence of a DC magnetic field. Under an external magnetic field, atoms in each domain will diffuse to form preferred atomic pairs with respect to the external field. Thus, a texture is established along the magnetic field direction which favors 180.degree. domain wall structure, and the magnetization process along this direction is easier than along other directions.
There are some weaknesses in static magnetic annealing. First, the ease of a domain wall displacement in a magnetization process along the easy direction is determined by the fluctuation of anisotropy energy along the path of the domain wall displacement. If the magnitude of the anisotropy, K.sub.u, is smaller, then the fluctuation of anisotropy will also be smaller. From this point of view, creating the texture with smaller directional-order-induced anisotropy is the original task. However, in the case of static magnetic annealing, the external magnetic field merely turns the direction of the directional order for different domains into a common direction (parallel to the external magnetic field) but does not reduce the magnitude of the anisotropy. This limits the improvement of magnetic properties by static magnetic annealing.
Second, the formation of the crystallographic as well as magnetic texture is due to the action of the magnetic field. Since the magnetic field is applied only in one dimension, the texture formed is one dimensional. The orientations of the 180.degree. domain walls in the transverse directions are still random.
Third, soft magnetic alloys are often fabricated in thin sheet shape in order to reduce the eddy current loss, and the magnetization process is along the longitudinal direction of the sheet. It is important to produce a planar texture such that it makes the domain walls parallel to the sheet plane. However, the domain structure obtained by static magnetic annealing in the interior of the sheet is not so. Therefore, there is a need for new methods of improving soft magnetic properties of crystalline and nanocrystalline magnetic alloys.
The related art is represented by the following patents of interest.
U.S. Pat. No. 3,963,533, issued on Jun. 15, 1976 to James D. Collins, describes a method of applying an alternating magnetic field to a ferromagnetic material after cooling the material in liquid nitrogen. Collins does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
U.S. Pat. No. 4,312,683, issued on Jan. 26, 1982 to Hiroshi Sakakima et al., describes a method of heat-treating amorphous alloy films having Curie temperatures higher than their crystallization temperatures in the presence of directed magnetic fields. Sakakima et al. do not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
U.S. Pat. No. 4,379,004, issued on Apr. 5, 1983 to Yoshimi Makino et al., describes a method of heat treating an amorphous magnetic alloy under an application of a magnetic field in which the direction of the applied magnetic field and the alloy are relatively rotated with respect to each other. Makino et al. do not suggest the use of an elliptic-polarized magnetic field, an oscillation magnetic field, or a pair of pulsed magnetic fields. Makino et al. do not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
U.S. Pat. No. 4,473,415, issued on Sep. 25, 1984 to Yoshitaka Ochiai et al., describes a method of heat-treating an amorphous magnetic alloy under an application of DC magnetic fields applied in two perpendicular directions. Ochiai et al. do not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
U.S. Pat. No. 4,475,962, issued on Oct. 9, 1984 to Masatoshi Hayakawa et al., describes a method of heat-treating an amorphous magnetic alloy under an application of a repetition of alternately applied first and second magnetic fields. The applied first and second magnetic fields have the same magnitude which may result in undesirable magnetic properties. Hayakawa et al. do not suggest the use of an elliptic-polarized magnetic field, an oscillation magnetic field, or a pair of pulsed magnetic fields. Hayakawa et al. do not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
U.S. Pat. No. 4,575,695, issued on Mar. 11, 1986 to Ernst F. R. A. Schloemann, describes an arrangement capable of applying a first and second magnetic fields along first and second directions. Schloemann does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
U.S. Pat. No. 4,816,965, issued on Mar. 28, 1989 to Vladimir Drits, describes an arrangement for providing a pulsed magnetic field. Drits does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
U.S. Pat. No. 5,032,947, issued on Jul. 16, 1991 to James C. M. Li et al., describes a method of improving magnetic devices by applying AC or pulsed current. Li et al. do riot suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
European Patent document number 0 027 362, published on Apr. 22, 1981, describes a method of improving magnetic properties of a magnetic material by subjecting the material to a magnetic field while applying mechanical vibrations or a high energy corpuscular beam to it. European document '362 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
German Patent document number 224,994, published on Jul. 17, 1985, describes a method of reducing the magnetic impedance of a magnetic core by applying a pulsed magnetic field before and during fixing. German document '994 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
Great Britain Patent document number 2,088,415, published on Jun. 9, 1982, describes a method of heat-treating an amorphous magnetic alloy under an application of a magnetic field while effecting relative rotation between the magnetic field and the alloy. British document '415 does not suggest the use of an elliptic-polarized magnetic field, an oscillation magnetic field, or a pair of pulsed magnetic fields. British document '415 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
Japan Patent document number 56-37609, published on Apr. 11, 1981, describes a method of producing a magnetic head core material with the application of a rotating magnetic field. Japanese document '609 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
Japan Patent document number 57-114646, published on Jul. 16, 1982, describes a method of heat-treating an amorphous magnetic material with the application of a rotating magnetic field. Japanese document '646 does not suggest the use of an elliptic-polarized magnetic field, an oscillation magnetic field, or a pair of pulsed magnetic fields. Japanese document '646 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
Japan Patent document number 59-35431, published on Aug. 28, 1984, describes a method of heat-treating an amorphous ferromagnetic alloy with the application of a rotating magnetic field. Japanese document '431 does not suggest the use of an elliptic-polarized magnetic field, an oscillation magnetic field, or a pair of pulsed magnetic fields. Japanese document '431 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
Japan Patent document number 63-290219, published on Nov. 11, 1988, describes a method of heat-treating an amorphous magnetic material with the application of a rotating magnetic field. Japanese document '219 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
Soviet Union Patent document number 394,164, published on Aug. 22, 1973, describes a method of sintering metal-ceramic parts using a diverting system of two electromagnets creating crossed magnetic fields. Soviet document '164 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
Soviet Union Patent document number 959,925, published on Sep. 23, 1982, describes a method of applying a layer of metal powder to a base made of compact material by forming and heating, with treatment after heating by a pulsed magnetic field. Soviet document '925 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
Soviet Union Patent document number 1,027,782, published on Jul. 7, 1983, describes an arrangement useful in the manufacture of permanent magnets. Soviet document '782 does not suggest annealing crystalline or nanocrystalline magnetic alloys in a dynamic magnetic field according to the claimed invention.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.